Mechanisms governing the relation between activity in the cerebral cortex and level of arousal

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Abstract
1. It has been shown that the spontaneous activity of many neurons in the cerebral cortex changes whenever the animal's level of arousal alters (A. C. Webb, Proc. R. Soc. Lond. B 194, 225-237, 239-251, 1976). It is generally agreed that the temporal pattern of discharge is sensitive to fluctuations in level of arousal. There is less agreement concerning effects on discharge frequency. 2. We have re-examined our own records of the spontaneous discharge of single cells in the visual, parietal and auditory cerebral cortex of unanaesthetized and unrestrained cats. We have also searched the literature for all comparable observations of single neurons in various cortical sites, recorded in the same and in other species. Most of the available data seem consistent with the idea that mean frequency of discharge of all mammalian cortical cells is affected in the same way by the transition from waking to slow-wave, or motionless sleep. 3. If the mean frequency of a cortical neuron when the animal is awake be W/s, and that of the same neuron in the sleeping animal be S/s, then the data from various cortical sites in cat and monkey can be fitted by a straight line: S = 2.00+0.45 W, correlation coefficient = 0.64. 4. The changes in discharge frequency that occur when an alarmed animal relaxes can also be fitted by a straight line. If the mean frequency of a cortical neuron when a cat is peacefully awake be W/s and that of the same cell when the cat is alarmed be A/s, then the relation between W and A can be described by W = 0.92+0.84 A, correlation coefficient = 0.92. 5. Neither excitation nor inhibition alone can account for the changes in spontaneous activity that accompany shifts in level of arousal. We propose a simple neural model that can account for the observed changes. This model assumes that, as level of arousal falls, the output of some subcortical system increases, providing both excitation and inhibition in fixed proportions.